This is followed by acetic acid fermentation in which acetic acid bacteria oxidise ethanol into acetic acid, imparting the characteristic sourness and complex flavour profile.

Most importantly, it’s a superstar when it comes to fermentation – producing lactic acid, acetic acid and carbon dioxide during the process.

These bacteria are known as acetic acid bacteria because they convert the alcohol generated by the yeast into acetic acid. The acid soaks into the beans, causing biochemical changes.